A large number of photosensitive materials are known which have a photosensitive layer on a support and form image by imaging exposure. An example of a system that enables environmental conservation or simplification of image formation includes a technique of forming an image by heat development.
In recent years, reduction of amount of waste processing solutions is strongly desired in the medical diagnosis field and the photographic art field from the standpoints of environmental protection and space savings. Techniques relating to photothermographic materials for use in the medical diagnosis field and the photographic art field are required which enables efficient exposure by a laser scanner or a laser image setter and formation of a clear black image having high resolution and sharpness. These photothermographic materials can provide users with a simple and non-polluting heat development processing system which eliminates the use of solution-type processing chemicals.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klosterboer, Imaging Processes and Materials, "Thermally Processed Silver Systems", Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). These photothermographic material contain a reducible light-insensitive silver source (e.g., organic silver salt), a photocatalyst (e.g., silver halide) in a catalytically active amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photothermographic material is stable at an ambient temperature, but when the material is heated at a high temperature (e.g., 80.degree. C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the reducible silver salt in the exposure region provides a black image and this presents a contrast to the non-exposure region to form an image.
In the thermally processed photographic materials, which contains the photothermographic material and the thermographic material, the formation of fog is a large problem. Many attempts have been made for reducing fog of the thermally processed photographic materials, and for example, U.S. Pat. No. 3,589,903 discloses mercury salt. In other examples, U.S. Pat. No. 4,152,160 discloses carboxylic acids such as benzoic acid and phthalic acid, U.S. Pat. No. 4,784,939 discloses benzoylbenzoic acid compounds, U.S. Pat. No. 4,569,906 discloses indane and tetralinecarboxylic acid, U.S. Pat. No. 4,820,617 discloses dicarboxylic acids, and U.S. Pat. No. 4,626,500 discloses heteroaromatic carboxylic acids. U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946 and 3,955,982 disclose halogenated compounds. Furthermore, U.S. Pat. No. 5,028,523, discloses halogen molecules and a halogen atom bonded to a hetero-atom ring. U.S. Pat. No. 4,103,312 and British Patent 1,502,670 disclose palladium compounds, U.S. Pat. No. 4,128,428 discloses iron family metals, U.S. Pat. Nos. 4,123,374, 4,129,557, and 4,125,430 disclose substituted triazoles, U.S. Pat. Nos. 4,213,784 and 4,245,033 and Japanese Patent Laid-Open No. 26019/1976 disclose sulfur compounds, U.S. Pat. No. 4,002,479 discloses thiouracils, Japanese Patent Laid-Open No. 123331/1975 discloses sulfinic acid, U.S. Pat. Nos. 4,125,403, 4,152,160 and 4,307,187 disclose the metal salts of thiosulfonic acid, Japanese Patent Laid-Open Nos. 20923/1978 and 19825/1978 disclose the combinations of metal salts of thiosulfonic acid and sulfinic acid, and Japanese Patent Publication No. 50810/1987 and Japanese Patent Laid-Open Nos. 209797/1995 and 43760/1997 disclose thiosulfonic acid esters.
Japanese Patent Laid-Open No. 42529/1976 and Japanese Patent Publication No. 37368/1988 disclose disulfide compounds. However, these compounds have the faults that the effect of preventing the formation of fog is low and when the addition amount is increased, Dmax (the maximum density) and the sensitivity are lowered.
Furthermore, in the photothermographic materials, polyhalogen compounds are very effective components as antifoggants and storage stabilizers, and are disclosed, for example, in Japanese Patent Publication No 165/1979, EP-605981A, EP-631176A, and U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946, and 3,955,982. However, the compounds described therein have the problems that the effect of preventing the formation of fog is insufficient, the storage stability of the photothermographic materials before development is insufficient, and the image storage stability (e.g., coloring of non-imaged portions by heat or light) after heat development is insufficient, and also have the problems that when the compounds is added in such an amount that sufficiently prevents the formation of fog, the sensitivity is lowered and Dmax is lowered.